REPRINT 1998
n e e Physiology 3, 193-202 (1987).
:tOF
6 1987 Heron Publishing-Vicroria, Canadn
0 0 1 1 6 : .32
Gas exchange and dry matter allocation responses
to elevation of atmospheric C 0 2 concentration in seedlings
of three tree species
D. Y. HOLLINGER
Forest~yResearch Cerlrre. Foresr Research Inslitule. P.O. Bo.r 31-011. Chri.rtc.lrurc.h.Neli Zetrl~rnrl
Received January 2, 1987
Summary
Photosynthetic rates of 13-month-old Pinus rndiarn D. Don, Notf~ofngusfusca (Hook f.) 0rst. and
Pseudofsuga menziesii (Mlrb.) Franco seedlings grown and measured at elevated atmospheric concentrations of C 0 2 (-620 pI I -') were 32 to 558 greater than those of seedlings grown and measured at
ambient (-310 )11 I-') concentrations of CO?. Seedlings grown in ambient and elevated concentrations of COz had similar rates of photosynthesis when measured at -620 ~1 I-' COz, but when
measured at -310 p1 I-' C 0 2the P. rcldiata and N. fusca seedlings which were grown at elevated COz
had lower rates of photosynthesis than the seedl~ngsgrown at an ambient concentration of CO?.
Stomatal conductances in general were lower when measured at -620 p,l I - COz than at -3 10 )*II
coz.
Stomatal conductances declined In all species grown at both C 0 2 concentrations when the leaf-air
water vapor concentration gradient (AW) was Increased from 10 to 20 mmol H1O mol-I alr. The
percent enhancement in photosynthesis for P. radiara and P. merciesii. at elevated C02was greater at
20 mmol than at 10 mmol AW, suggesting that elevated CO1 may moderate the effects of atmospheric
water stress.
Dry matter allocation patterns were not s~gnificanrlydifferent for plants grown in ambient or high
COz air.
Introduction
Short-term increases in COr concentration increase assimilation rate and decrease
transpiration rate in C3 plants (e.g., see review by Rarcy and Bjorkman 1983).
More detailed knowledge of the effects of elevated concentrations of CO, on tree
species, however, is limited. Areas of uncertainty include an inability to predict
the magnitude of the response of species not yet investigated, the long-term effects
of elevated atmospheric COz concentration, and the way in which the effects of
elevated COr concentrations interact with other environmental variables.
Several workers have investigated the proportionality between increases in
photosynthetic rate and atmospheric COr concentration (enhancement ratio) in tree
seedlings. The ratio of enhancement for seedlings grown and measured in the 300
to 700 pl I - ' COz range is from less than 0.4 to more than 0.75 (Carlson and
Bazzaz 1980, Wong 1980, Rogers et a1. 1983, Tolley and Strain 1985, Williams et
al. 1986) and has no apparent relation to phylogeny or growth form.
Some studies suggest that prolonged exposure to elevated CO, concentrations
may reduce plant photosynthetic capacity at ambient concentrations of CO, (e.g.,
HOLLINGER
Frydrych 1976, Wong 1980). Extrapolations of photosynthesis made from measurements on plants grown at ambient concentrations of CO? to plants grown at
elevated CO2 must therefore be made with caution.
Light, soil water content and soil nutrient status all modify the effects of
elevated C02 on seedling gas exchange (Wong 1980, Tolley and Strain 1985,
Williams et al. 1986). For example, the greatest relative enhancement of photosynthesis occurs when light intensity is high or the availability of soil water is low.
The effects of some environmental factors such as the leaf-air water vapor concentration gradient, AW, on the photosynthetic response to elevated COz concentrations are unknown.
In this study, with seedlings of two conifers, Pinus radiatn D. Don and Pseudotsuga menziesii (Mirb.) Franco, and one hardwood species, Nothofagus fusca
Hook f.) (drst., I investigated three questions. (1) The relationship between shortand long-term responses to elevated atmospheric CO. concentration. (2) The interaction between COz concentration and leaf-air water vapor concentration difference (AW) in their effects on transpiration and photosynthetic COz assimilation.
(3) The influence of atmospheric COz concentration on the production and allocation of dry matter.
Methods
Nine-month-old, nursery stock of Monterey pine (Pinus radiata), Douglas-fir
(Pseudotsuga rnenziesii), and New Zealand red beech (Nothofagus fuscn) were
transplanted during the winter into 4-liter containers filled with a 1:1 mixture of
sand and vermiculite. The transplanted stock remained outdoors for a further six
weeks. Plants of each species were ranked in eight groups of three on the basis of
stem diameter. One plant from each group was randomly assigned to each of three
sets. One set was harvested immediately for dry weight determinations and the
other two sets of eight plants were placed in separate environmentally controlled
growth cabinets (Temperzone Ltd., Auckland).
Growth conditions for these two groups were identical except for daytime CO?
concentration, which was 340 + 20 p1 1 - I for the control plants and 640 ? 30 pl
1-I for the high CO2 treatment. Daylnight temperatures and relative humidities
were 20110 "C and 70190% RH, respectively. Photoperiod was 14 hours, with a
photosynthetically active photon flux density (PPFD) of 700 + 50 pmol m-' s - I
at the top of the canopy provided by mercury halide and tungsten-halogen lamps.
Plants were watered with one-half strength Hoagland's solution three to four times
per week. Gas exchange measurements and dry weight analyses were made on the
plants after 120 days in the cabinets.
Chamber CO2 concentration was controlled with a system consisting of an
infrared gas analyzer (ADC Mk-3, Analytical Development Company, Hoddesdon, England), gas sampling and switching system, solenoid valves, and controller. This system injected pure COz (underground origin) whenever the carbon
dioxide concentration dropped below the set values of 340 and 640 p1 1-I.
GAS EXCHANGE, GROWTH AND C02 CONCENTRATION
Gas exchange of one to three needles or leaves per plant was measured with an
open system similar to that described by Schulze et al. (1982). Measurements
were made inside the controlled environment chamber through reach-in ports with
an unclimatized (without temperature control), clip-on acrylic plastic cuvette
(internal volume approximately 5 ml) instead of the porometer head described by
Schulze et al. The CO2 concentration of the airstream was controlled by mixing
compressed air from a cylinder with 1% C02 in N2 by means of mass flow
controllers (Tylan Corporation, Carson, California, USA), and was humidified by
means of a water vapor generator (Model WG-600, Analytical Development Company). Leaf and cuvette airstream temperatures were measured with chromelalumel thermocouples with an electronic reference (Model MCJ, Omega Engineering, Inc., Stamford, Connecticut, USA). Projected leaf areas were measured
with a video leaf area meter (Delta-T Devices, Ltd., Cambridge, England) and
converted to total leaf area. A flow rate of approximately 300 ml min-I meant that
equilibrium conditions were achieved in less than 30 seconds after the cuvette was
attached to the foliage.
Gas exchange measurements on a single set of leaves or needles from each plant
were made on successive days at COz concentrations of approximately 310 and
620 pl I-' CO2, with a leaf-air water vapor concentration difference (AW) of
approximately 10 mmol H20 mol-I air. The same foliage was also measured at the
concentration of CO2 maintained during growth and with a AW of approximately
20 mmol Hz0 mol-1 air. All measurements were made at 20 + 1 "C with a PPFD
of 700-800 pmol m-2 s-I. Six hours before gas exchange measurements, conditions of CO2 concentration and relative humidity in the growth cabinets were
adjusted to approximate those to be maintained in the cuvette. Photosynthesis,
transpiration and conductance values were calculated on the basis of total leaf area
(Cowan 1977).
Following the gas exchange measurements, plants were harvested and divided
into six components; new leaves or needles (foliage produced while the plants
were in the growth cabinets), old leaves or needles, new stems, old stems, roots <
2 mm in diameter, and roots > 2 mm in diameter. The material was dried in a
forced draft oven to constant weight at 80 "C.
Results of the gas exchange and harvest measurements were analyzed with
repeated-measures analysis of variance. Significant differences between growth or
measurement conditions were determined by F-tests.
Results and discussion
Growth and allocation
Dry weight of all species in both treatments increased approximately three- to
fivefold during the 120-day experiment (Table 1). Dry weights of Pinus radiata
and Nothofagus fusca seedlings grown at 640 p.1 I-' CO2 were significantly greater
at harvest than those of control plants (Table 1). There was no significant effect of
COz concentration during growth on dry weight of Pseudotsuga menziesii perhaps
Table 6. Percent change (mean and standard deviation) in foliage photosynthesis (A) and conductance (g) with a step increase in AW from 10 to 20 mmol Hz0 mol-I
air.
Percent change with a
10 mmol H20mol air increase in AW
Norhofagus fusca
Pinus radiara
Ambient COz
Elevated COZ
Ambient COZ
Pseudotsuga menziesii
Elevated C02
(* = Response of plants grown and measured at approximately 620 )11 I - ' COz significantly different (P
measured at approximately 310 )11 I-'.)
Ambient C02
Elevated C@
< 0.05, r-test) from plants of same species grown and
HOLLINGER
Table 1. Dry weights of tree seedlings (g per seedlings, mean and standard deviation).
Species
P. radia~a
N. fusca
P. menziesii
Before treatment
1 1 . 1 (3.3)
1.3 (0.4)
4.5 (1.6)
After 120 days growth
330 pl I ' COz
630 p1 I-' COZ
46.4 (13.1)
4.2 ( 1 .O)
18.5 ( 8.1)
59.1 (12.9)"
4.9 ( 1.9)*
19.0 ( 5.4)
(* = Mean significantly different from control (330 p.1 I-') value, P
< 0.05, repeated measures
F-test.)
because of the unexpected variability in seedling vigor.
The allocation of dry matter within the plant varied among species (Figure 1).
Roots made up a significantly higher percentage of total biomass in N. fusca than
in P. radiata or P. menziesii. In P. radiata and P. menziesii the allocation to fine
roots increased significantly during the 120-day growth period at both control and
elevated CO2 concentrations, which suggests that nutrient availability may have
been low. The allocation of biomass between root and shoot or between individual
structures (e.g., foliage, stems) did not change with CO2 growth concentration for
any species (Figure 1). The observed variation in allocation percentages within
species was low. There was no significant difference between COr treatments in
leaf specific weights (data not shown).
Growth at elevated concentrations of COz does not change gross allocation in
Acer saccharinum L., Populus deltoides Bartr. ex Marsh., or Platanus
occidentalis L. (Carlson and Bazzaz 1980). The evidence for Liquidambar
styracij7ua L., however, is conflicting. Sionit et al. (1985) found no difference in
root/shoot ratios of seedlings grown for one year at 350, 500, or 650 p1 I-' COz.
However, Tolley and Strain (1984) found that root/shoot ratios of Liquidambar
seedlings grown for 90 days at 675 and 1000 yl 1 - I were significantly lower than
those of seedlings grown at 350 p1 I-', whereas Rogers et al. (1983) found that the
root/shoot ratio of Liquidambar grown for about 90 days at 910 p1 I-' CO2 was
significantly higher than ratios from plants grown at ambient levels of CO2. In all
of these investigations the seedlings were grown in pots.
Foliage photosynthesis and conductance
Photosynthetic rates and stomata1 conductances both differed significantly among
species, the conifers having the highest values (Tables 2 and 4).
For each species, photosynthetic rates were significantly higher when measured
at 620 p1 I-' CO2 than when measured at 310 p1 1-' C02 (Tables 2 and 3). The
enhancement of photosynthesis as a proportion of the increase in concentration of
C02 of plants grown and measured at ambient (about 310 p1 I-' CO2) versus -620
~1 1 - I C02 was about 0.55 for P. radiata, 0.45 for N . fusca, and 0.32 for P.
menziesii. These enhancement ratios are similar to those obtained for seedlings of
other tree species (e.g., Rogers et al. 1983, Williams et al. 1986) and will likely
GAS EXCHANGE, GROWTH AND C o t CONCENTRATION
Pinus r a d i a t a
95% C o n f i d e n c e
Interval
Pseudotsuga menziesii
Nothofagus fusca
401-
Foliage
Stems
Fine Roots
C o a r s e Roots
Figure I. Dry matter allocation In tree seedlings. Elevated CO1 concentrations during growth did not
change allocation m any species.
vary somewhat depending on nutrient or water availability during growth (Sionit
et al. 1980, Wong 1980, Tolley and Strain 1985).
The concentration of COz in which the plants were grown had no simple effect
on photosynthetic rates (Tables 2 and 3). The ANOVA table (Table 3) shows that.
in P. radiata and N. fusca, there was a significant interaction between growth and
measurement concentrations of COr on photosynthetic rates. Inspection of Table 2
suggests that growth concentration of C02 had no effect on photosynthesis measured at 620 p1 l-' CO2, but that when measurements were made at 310 p1 1-1
COz, plants grown in elevated concentrations of COz had lower photosynthetic
rates than plants grown in ambient concentrations of COz. This decrease in photosynthetic capacity at ambient concentrations of COr in plants grown at elevated
concentrations of C02 has been 05served in other species (Hofstra and Hesketh
HOLLINGER
Table 2. Photosynthetic rate of tree seedlings grown at 340 and 640 p1 I - ] C 0 2 (kmol COz m-* s-I,
mean and standard deviat~on)].
Concentration
of COz
during growth
( d l - '1
I
Concentration of COz during measurement (p1 I-')
620
3 10
Pinus radrata
310
620
Nothofagus fusca
Measurement temperature = 20 k 1 "C, PPFD = 750
H20mol - air.
k
310
620
Pseudoisuga menzresir
50 pmol m-'s-l, AW = 10
* 1 mmol
Table 3. Analysis of variance of photosynthetic rates
DF
Source
F
Prob.
P. radiata
Growth C 0 2
Measurement CO,
Interaction
N . fusca
Growth CO,
Measurement C 0 2
Interaction
P rnenziesii
Growth C 0 2
Measurement COz
Interaction
Table 4. Stomata1 conductances of tree seedlings grown at 340 and 640 p1 I-' CO, (mmol m-Is-I,
mean and standard deviat~on)~.
Concentration
of COz
during growth
Concentration of CO, during measurement (pl I-')
310
620
Pinus radiata
-
I
310
620
Nothofagus fusca
310
620
Pseudotsuga menziesir
-
Measurement conditions same as for Table 2.
1975, Imai and Murata 1978, Wong 1979, 1980, DeLucia et al. 1985). Several
mechanisms have been proposed, including feedback inhibition due to starch
accumulation (Nafziger and Koller 1976), a reduction in inorganic phosphate
concentration in the chloroplasts (Herold 1980), an increase in carbonic anhydrase
activity (Chang 1975), or a decrease in ribulose-1,5-bisphosphate carboxylase
activity (Wong 1979). The results presented here, as well as those of others (e.g.,
Wong 1979, 1980, Higginbotham et al. 1985, but not DeLucia et al. 1985),
suggest that in some cases there is a compensation for the reduction of photo-
GAS EXCHANGE, GROWTH AND C 0 2 CONCENTRATION
synthetic capacity of plants grown at elevated CO2 concentrations so that photosynthetic rates at high concentrations of CO2 are similar to those of plants grown at
ambient concentrations but measured at high concentrations. Wong speculated that
the compensation was effected by an increase in chloroplast photochemical activity of plants grown at elevated COz. A result of this compensation is that shortterm experiments with seedlings grown in ambient air measured at elevated levels
of CO2 may accurately reflect the response of the same material grown and
measured at elevated concentrations of CO2.
The concentration of CO2 during plant growth had no simple effect on stomatal
conductance (Tables 4 and 5). However, conductances were generally lower when
measured at 620 p.1 1-1 than at 310 p.1 I-' CO2. In Pinus radiata, stomatal
conductances of plants grown in ambient concentrations of C 0 2 were similar when
measured at 310 or 620 p.1 1 - I CO2. Conductances of P. radiata grown at 640 p1
1-I C02, however, were lower when measured at 620 than when measured at 310
p.1 I-' CO2, which suggests that the sensitivity of the stomata to CO2 increased in
plants of this species grown in COz-enriched air. For all species and growth
treatments, the mean decrease in conductance with a doubling of the CO2 concentration during measurement was about 14%. As a result of the combined effects of
increased photosynthesis and decreased conductances, the water use efficiency
(moles CO2 assimilated/moles H20 transpired) of plants grown and measured at
elevated CO:! was greater than that of plants grown and measured at ambient
concentrations of C02. This increase was about 73% in P. radiata, 63% in N .
fusca, and 36% in P. menziesii.
Effect of leaf-air water vapor concentration gradient
Photosynthetic rates and stomatal conductances decreased significantly in all species and for both CO2 growth treatments when AW was increased from about 10
mmol H20 mol- air to 20 mmol Hz0 mol-' air (Table 6). The response of the
conifers was somewhat greater than that of N. fusca.
Table 5 . Analysis of variance of conductance.
--
Source
DF
F
Prob.
P. radiara
Growth COz
Measurement COz
Interaction
1
1
0.83
7.35
1
8.67
0.620
0.016
0.010
N.fusca
Growth C02
Measurement C 0 2
Interaction
P.menziesii
Growth C 0 2
Measurement C 0 2
Interaction
GAS EXCHANGE, GROWTH AND COz CONCENTRATION
In Pinus radiata and Pseudotsuga menziesii, COz concentration modified the
effects of AW. The relative stomatal closure induced by a step change in AW in
these species was less at the elevated CO2 concentration than at the ambient CO2
concentration. Carbon dioxide concentration did not alter the effect of AW in
Nothofagus fusca. In the conifers, however, C02 concentration apparently
modifies the gain of the stomata1 feedforward response to AW. Because there is
less relative stomatal closure at an elevated concentrations of COz, the relative
enhancement of photosynthesis associated with an increase in COa concentration
is greater at a large AW than at a small AW.
Elevated concentrations of COz have been shown to ameliorate the effects of
soil water stress on growth and photosynthesis in wheat (Gifford 1979, Sionit et
al. 1980) and Liquidambar (Tolley and Strain 1984, 1985). The results for Pinus
radiata and Pseudotsuga menziesii suggest that for these species, elevated CO2
concentrations may also moderate the effects of atmospheric water stress.
Acknowledgments
U. Benecke and I. McCracken assisted with the gas exchange measurements and with J. Onvin
provided valuable comments on the manuscript. A. Allan and J. Byers provided expert technical
support.
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